A conventional mechanism for changing the rotary transmitting channel in an auto-reverse tape recorder is disclosed in a Japanese Patent Application Laid Open No. Sho 60-113,349. In this conventional mechanism, when a slide plate 1 is in the left-hand position as shown in FIG. 3, a right change-over gear 4 is meshed with a small-diameter gear 5.sub.1 of a right transmission gear 5 by means of the clockwise rotation of a right rotatable plate 2 due to the tension force of a tension spring 3, while a left change-over gear 7 is separated from a small-diameter gear 8.sub.1 of a left transmission gear 8 by means of the clockwise rotation of a left rotatable plate 6 due to the compressive force of the tension spring 3. Moreover, since a pinch roller change-over plate 9 is inclined to the left by means of a cam pin 10, a right pinch roller 12 on a right end of a rod-shaped spring 11 is passed to, and made contact with, a right capstan 13, and at the same time, a left pinch roller 14 on a left end of the rod-shaped spring 11 is separated from a left capstan 15. Accordingly, the turning force of the motor (not shown) is transmitted to a right reel support 17 through the right transmission gear 5, the small-diameter gear 5.sub.1, the right change-over gear 4 and a right reel gear 16, and therefore a tape (not shown) travels to the right through the right pinch roller 12 and right capstan 13 and then wound on a right reel (not shown).
On the other hand, when the slide plate 1 is in the right-hand position as shown in FIG. 4, the right change-over gear 4 is separated from the small-diameter gear 5.sub.1 of the right transmission gear 5 by means of the counterclockwise rotation of the right rotatable plate 2 due to the compressive force of the tension spring 3, while the left change-over gear 7 is meshed with the small-diameter gear 8.sub.1 of the left transmission gear 8 by means of the counterclockwise rotation of the right rotatable plate 6 due to the tension force of the tension spring 3. Further, since the pinch roller change-over plate 9 is inclined to the right by means of the cam pin 10, the right pinch roller 12 on the right end of the rod-shaped spring 11 is separated from the right capstan 13, and at the same time, the left pinch roller 12 on the left end of the rod-shaped spring 11 is pressed to, and made contact with, a left capstan 15. Thus, the turning force of the motor is transmitted to a left reel support 19 through the small-diameter gear 8.sub.1 of the left transmission gear 8 engaged with a large-diameter gear 5.sub.2 of the right transmission gear 5, the left change-over gear 7 and the left reel gear 18, and therefore the tape (not shown) is traveled to the left through the left pinch roller 14 and left capstan 15 and then wound on a left reel (not shown).
In a conventional mechanism for changing the rotary transmitting channel, there exists the disadvantage that the number of component parts thereof is increased and the construction thereof is complicated, because, as above-described, the mechanism is composed of a pair of change-over gears, a pair of complicated, rotatable plates for supporting the change-over gears, and the tension spring installed between two rotatable plates.